How Kliva sets fueling and pacing for trail races

The defaults follow published sports science, not brand marketing. Three sources do most of the work:

• Jeukendrup's reviews on carbohydrate intake during exercise: how much the gut can absorb, and why the answer depends on how long you are out.
• The ACSM position stand on exercise and fluid replacement: avoid large body-mass losses, and expect a wide range of sweat rates between runners.
• The ISSN position stands, including the one on ultra-endurance: steady drinking, enough sodium in the drink for very long events, and a different carbohydrate approach beyond many hours.

Where position stands are silent, practitioner data on sweat sodium and on common gels and drink mixes fills the gap.

The carbs-per-hour default is picked from the planned race duration. In principle:

• Short efforts, up to roughly an hour: water is enough, so the default is no carbohydrate.
• Around one to two and a half hours: a moderate amount from a single carbohydrate source.
• Roughly two and a half to six hours: the highest default. Intensity is still high and the gut can take more, which calls for a glucose and fructose mix.
• Beyond six hours: a little lower again, because the pace is lower and real food takes over from gels.

Amounts are absolute per hour, not per kilogram of body weight, because the limit is how fast the gut absorbs, not how big the runner is.

A runner can raise the target. Above the usual ceiling for the duration, Kliva asks whether the gut is trained for it and warns when it is not.

Fluid starts from the conditions you pick for the race (cool, moderate or hot), or from the runner's own sweat rate when it is known. Along the course it then moves with the air temperature:

• A weather forecast, when there is one, adds water where it is hotter than the plan assumed. Cold never takes water away.
• Altitude: air cools as you climb and sweat loss follows temperature, so the plan drinks a little less on high ground and a little more in valleys below the start.

Rates stay inside safe bounds, and a typed target above what the body can absorb gets a warning. Over-drinking is a real risk in slow ultras (hyponatremia), so more is not better.

Sodium is set as a concentration in the drink, so it rises and falls with the fluid plan: hot days and valleys mean more sodium per hour, cool days and high ground less.

Sweat sodium varies a lot between runners and is largely genetic. The default sits in the middle of published ranges, above the minimum the ISSN suggests for very long events, and can be adjusted for a salty or a light sweater.

Your target finish time is the anchor. Segment times spread that time over the course, so the last segment always ends exactly on target and the segments, aid stations and watch targets all agree to the minute.

Each segment is priced by effort, not distance. Its cost is its length plus the climbing in it, taken from the course GPX, so a short steep section counts for more than a long flat one.

Descents are not free. Gentle descents are faster than flat, very steep ones give most of that back, so a descent is never treated as zero effort.

People slow down. The plan shifts effort toward the first half so the second half takes longer, by an amount that grows with the target time. The model behind this is calibrated on nearly 1,000,000 race finishes.

Climbing, Descending and Split can be adjusted for each plan, so a strong climber or a runner who fades late can be modelled. Only the Split suggestion is fitted on finisher data; the Climbing and Descending levels are a coaching choice.

Limits: the course profile is simplified, so short steep pitches are averaged out, and the plan knows nothing about technical terrain or the runner's form. Treat segment times as a well-informed pacing guide, not a prediction.

Build a race plan to see these defaults applied to a real route.

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